What the connection between IDL and GMT?

IDL=International Date Line and GMT =Greenwich Mean Time; Greenwich, near London, was used to set the "Prime Meridian" of 0 degrees running from the North to the South Poles. The time zone (each zone is usually roughly 15 degrees of longitude) for the UK is known as Greenwich Mean Time. Since these lines of longitude (meridians)number 360 around the globe, the 180 degree line falls in the middle of the Pacific Ocean. This turns out to be very convenient for time (date) keeping, as this meridian in a relatively uninhabited part of the Pacific forms the International Date Line (with some local variations). Crossing the IDL means you are moving from GMT+12 to GMT-12, or vice versa, which also means you are either moving behind or ahead a calendar day. -see http://www.wisegeek.com/what-is-the-international-date-line.htm